In general, an alkaline earth metal salt of aromatic hydroxycarboxylic acid is also called an alkaline earth metal salicylate and is used as an additive to a lubricating oil for internal combustion engine (engine oil). It exhibits marked effects for the neutralization of acids such as oxyacid and sulfuric acid, dispersion of sludge, lacquer, carbon and the like, and prevention of corrosion and abrasion, choking of ring groove, hang-up of piston and the like. The alkaline earth metal salt of aromatic hydroxycarboxylic acid or a sulfurized mixture thereof is excellent in heat stability so that it can maintain its activity as a detergent for a long period of time. Furthermore, the alkaline earth metal salt of aromatic hydroxycarboxylic acid is used as an additive for a lubricating oil, and, in addition, it is added in a small amount to a diesel fuel oil or gasoline fuel oil. The object of the addition to such a fuel oil is to prevent adhesion of sludge, carbon and the like to a combustion chamber.
The alkaline earth metal salt of aromatic hydroxycarboxylic acid, which is a main component of an alkali earth metal salicylate, has heretofore been prepared by a complex process, for example, by forming an alkaline metal salt of alkyl phenol, carboxylating the resulting salt in accordance with Korbe-Schmitt reaction, neutralizing the alkali metal salt of aromatic hydroxycarboxylic acid thus obtained with an inorganic acid and then reacting the neutralized salt with an alkaline earth metal compound or by conducting double decomposition using calcium chloride or the like. However, these processes contain many and complex steps including a rinsing step for completely removing strong electrolytes such as strong acid, strong alkali or chloride which promote corrosion of a metal such as engine. Moreover, even by such a complex treatment, strong electrolytes cannot have been removed completely.
On the other hand, the present inventors has found that an alkaline earth metal salt of aromatic hydroxycarboxylic acid can be formed directly by reacting an alkali earth metal salt of a phenol with carbon dioxide and discloses this method JP-A-61-282336 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-B-4-25996 (the term "JP-B" as used herein means an "examined published Japanese patent publication"). That is, it is a process for producing an alkaline earth metal salt of aromatic hydroxycarboxylic acid, which comprises very simple steps, conducting direct metal addition of an alkaline earth metal reagent to an alkyl phenol as a raw material and treating the resulting product with a carbon dioxide gas.
This process is an industrially advantageous process which makes it possible to obtain an alkaline earth metal salt of aromatic hydroxycarboxylic acid far easier than the conventional process, because it requires neither formation nor decomposition of the alkaline metal salt during the process and additionally, it does not require a rinsing step of a strong electrolyte.
Since alkaline earth metal salt of aromatic hydroxycarboxylic acid has drawbacks in oil solubility and storage stability, a phenol as a raw material is desired to have a hydrocarbon side chain having at least 15 carbon atoms, preferably at least 18 carbon atoms.
However, since a phenol having such a long-chain hydrocarbon as a side chain is more expensive than a general-purpose phenol having 12 or less carbon atoms as a side chain, it is difficult to use the former phenol from the economical viewpoint. Furthermore, even if such a long-chain phenol is employed, it is not free from a drawback in stability upon long time storage.
A phenol is the most expensive material among the raw materials employed so that it is the common practice to recover a free phenol, which has remained in the reaction mixture, by distillation. It is however difficult to recover a long-chain phenol containing a hydrocarbon side chain having at least 15 carbon atoms, because its boiling point is high. Even if it is possible to recover by distillation, not only large utilities are required for the recovery but also the objective alkaline earth metal salt of aromatic hydroxycarboxylic acid is partially decarboxylated owing to the exposure to high temperatures not lower than 230.degree. C.
It is therefore not recommendable to use a phenol containing a hydrocarbon side chain having at least 15 carbon atoms as a raw material. Accordingly, there has been a demand for the development of a petroleum additive which contains an alkaline earth metal salt of aromatic hydroxycarboxylic acid--for which a phenol containing a hydrocarbon side chain having not more than 15 carbon atoms, preferably not more than 12 carbon atoms, is used as a raw material--and which is excellent in oil solubility and storage stability. When a phenol containing a hydrocarbon side chain having 15 or less carbon atoms is used in the conventional process, the alkaline earth metal salt of aromatic hydroxycarboxylic acid, which is the reaction product, inevitably has problems in oil solubility and storage stability.
It is known to conduct sulfurization of an alkaline earth metal salt of aromatic hydroxycarboxylic acid as a means for overcoming the above-described problems (JP-A-64-29354). If the sulfurization is conducted prior to carboxylation, however, an active portion of the phenol nucleus is consumed by sulfurization, which makes it difficult to conduct carboxylation. The sulfurization after carboxylation, on the other hand, inevitably causes elimination of one portion of the intentionally-introduced carboxyl group owing to the heat required for the sulfurization. This process is therefore accompanied with the problem that when oil solubility and storage stability are improved by sulfurization, the carboxylation ratio of the product largely lowers, leading to a deterioration in the heat stability, particularly, high-temperature cleanability (result of lacquer test) of the product.